Speedo calibration - revs/mile of 7.75-15 tire

[Patrick B.]

What are the revolutions per mile of a 7.75-15 tire? I assume that a measure of this tire accounting for tire deflection was the basis for the choice of speedo gears for 1967 Corvette transmissions. I am also assuming that the instrument was calibrated as 1000 rpm of the cable equals 60 mph. Can someone confirm these numbers? Thanks

[Duke W.]

Re: Speedo calibration - revs/mile of 7.75-15 tire

Loaded tire revs/mile are listed in the AMA specs and IIRC the 7.75-15 is speced at 774. I just call it 775, which is also the nominal loaded revs per mile for the F70-15 and 215/70R-15. The 6.70-15, GR70-15 and 225/70R-15 are nominally 760.

Typical "Web calculators" give incorrect answers because they just use the inflated OD to calculate revs per mile. This is what I call the "rigid body" revs/mile, which is fine for a steel railroad wheel, but doesn't take into account the deflection of a loaded pneumatic tire. Manufacturers actually measure the loaded revs per mile using a large drum. I believe they do this at max cold pressure and max load, but this should be fairly accurate for most real world situations. Tire manufacturer Web sites and the big online retailers usually have spec sheets for all make/model tires that include loaded revs/mile.

I've found from data I've accumulated over the years that if loaded revs per mile are not listed, increasing the rigid body calculation based on inflated OD by 3 percent will get you pretty close. Also keep in mind that from new to the wear bars, revs/mile increases by about 2 percent. Dimensions and actual revs per mile for any given size tire from any manufacturer should be very close if the manufacturer complies with Tire and Rim Association guidelines, which are within very narrow ranges and most manufacturers comply. One exception I've noted is the Avon CR6ZZ. The 205/70R-15 and 215/70R-12 have OD speced within 0.2" of each other, and the specified revs per mile appear to be based on the rigid body calculation.

Also I believe the 1000 RPM cable speed at 60 MPH is the GM spec for our vintage Corvettes with mechanical speedometers.

Duke

[Patrick B.]

[Duke W.]

Re: Speedo calibration - revs/mile of 7.75-15 tire

800 revs/mile x 3.36 = 2688 revs at 60

1000/2688 = 0.3720

8/22 = 0.3636 which would read 2.3 percent low or 58.65 at a true 60
8/21 = 0.3810, which would read 2.4 percent high or 61.45 at a true 60

So we differ on the reading for the 21T gear. By my calculation you can choose either slightly over two percent high or low. I'd probably go for 2.3 percent low since that will decrease with tire wear assuming you actually drive the car enough to wear the tread down significantly before you feel you should replace them due to age.

Duke

[Patrick B.]

Re: Speedo calibration - revs/mile of 7.75-15 tire

We only disagree in round off. I rounded the apparent speed with the 21 tooth gear to 61.4 rather than 61.45. But I will probably never wear the tires down much. The 62.5 mph figure was for the 3.70 rear and 7.75-15 tires.